D7100 Banding Issue...

As Falk Lumo has repeatedly emphasized, there is a random error in PDAF. It seems with the D7100 that these random errors may exceed the threshold of critical focus. The shots I get with the reticle right on the bird's eye that are OOF continue to puzzle me. You would think that like camera shake this issue would scale, but apparently not. I certainly don't have this problem with the D300/s. At any rate the more shots you get, the more likely you are to get shots where that random focus error isn't critical.

Camera shake relates to how many pixels/degree a shake motion comprises, and for equal MP count and sensor aspect ratios the shake sensitivity of cameras of differing sensor sizes should be about the same. But increasing the MP count will increase shake sensitivity.

I could easily see, however, where the OOF performance of a DSLR would get worse with increasing MP count...the sensor itself has nothing to do with the PDAF performance. You're still using that same separate AF sensor with its few thousand sample points and that same separate optical chain with maybe better processing, and those same lenses with the same mechanical limitations...so that flashy new high res sensor will resolve the random errors better.

This suggests that one of the reasons that Nikon went to the 51-pt AF sensor in the D7100 and beefed up the 35-point in the D5300 is because they had to (but then doesn't explain why they didn't with the D3300, except that perhaps its buyers wouldn't care...), but more that a jump in AF sensor resolution and/or area and of course lens mechanicals quality will be needed to continue MP scaling. Another reason why OSPDAF looks far more attractive if they can solve its own resolution/sensitivity/speed issues.

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